(1) Field of the Invention
This invention relates to boiling cooling systems suitable for effecting cooling of heat generating bodies by utilizing the boiling and condensing of a refrigerant, and more particularly it is concerned with a constant pressure type boiling cooling system capable of effecting cooling by maintaining the internal pressure of the system at atmospheric pressure level at all times.
(2) Description of the Prior Art
As semiconductor elements for use with a power system, such as thyristors, transistors, etc., have grown larger in capacity, boiling cooling systems have become popular for effecting cooling of these elements which utilize boiling and condensing of a refrigerant, such as Freon, to achieve excellent cooling function.
In many boiling cooling systems of the prior art that have hitherto been in use, the refrigerant in a liquid state is contained in a vacuum vessel for sealing the refrigerant therein. In this closed type of boiling cooling systems using a closed type refrigerant container, the internal pressure of the container varies from a condition which is substantially vacuum to a condition in which the pressure rises to about two atmospheric pressures depending on the thermal load applied thereto during cooling operations. This makes it essential to provide the container with a construction which could withstand such great fluctuations in its internal pressure. This would cause a large increase in cost.
To obviate the aforesaid disadvantages of boiling cooling systems of the closed type using a vacuum vessel, proposals have been made to use a constant pressure type boiling cooling system using a container for a liquid refrigerant which is provided with a liquid sump of a variable volume type utilizing bellows, for example, so that the system is able to operate while keeping its internal pressure substantially at the atmospheric pressure level regardless of changes in thermal load from room temperature to the maximum level. Such boiling cooling system has been put to practical use.
In the boiling cooling system of the aforesaid constant pressure type, when the evaporator has its temperature kept at a room temperature level, all the refrigerant passages including the interior of the condenser are filled with a refrigerant in a liquid state and the liquid sump of the variable volume type is in a condition of minimum volume. As the temperature of heat generating bodies, such as semiconductor elements, rises to apply a thermal load to the evaporator, the refrigerant in the evaporator boils and produces vapor by absorbing a large amount of latent heat of vaporization and the vapor condenses in the condenser to release a large amount of heat to the atmosphere, to thereby effect cooling. At this time, an increment of the volume of the refrigerant is cancelled out by an increase in the volume of the liquid sump of the variable volume type, so that cooling can be carried out at atmospheric pressure without an increase in internal pressure.
The boiling cooling system of the aforesaid construction offers the advantage that, since the internal pressure of the container including the evaporator and the condenser for the refrigerant to be sealed in can be kept at a level substantially equal to the atmospheric pressure irrespective of whether or not a thermal load is present, the need to use a pressure vessel is eliminated and the costs can be reduced.
In the constant pressure type boiling cooling system of the prior art, it is usual practice to arrange the evaporator and the condenser vertically so that the condenser is disposed above the evaporator to enable the refrigerant vapor produced in the evaporator to flow efficiently into the condenser.
When the cooling system of the aforesaid construction is used for cooling the semiconductor element referred to hereinabove, the evaporator and the semiconductor elements are assembled with each other in intimate contact condition, with the result that the evaporator, the semiconductor elements to be cooled by means of the evaporator and various electric accessory parts necessary for the semiconductor elements are located below the condenser.
Thus when the aforesaid constant pressure type boiling cooling system is applied to semiconductor elements, it would be necessary to efficiently pass a cooling air current from below toward the condenser located above the evaporator, making it impossible to effect cooling of the semiconductor elements satisfactorily with natural air currents. Even if forced circulation of air were relied on to obtain a satisfactory flow of cooling air, such measures would involve a rise in cost because a complex construction of an air passageway for guiding the cooling air current would make it necessary to increase the capacity of the blower to cope with an increase in the resistance offered to the flow of the cooling air.
Also, in the constant pressure type boiling cooling system of the prior art, the arrangement whereby the evaporator and the condenser are arranged vertically in superposed relation increases the vertical dimension of the semiconductor device in which the cooling system is incorporated, making it impossible to obtain a compact overall size in a semiconductor device.